For certain types of minimally invasive medical procedures, real time information regarding the condition of the treatment site within the body is unavailable. This lack of information inhibits the clinician when employing catheter to perform a procedure. An example of such procedures is tumor and disease treatment in the liver and prostate. Yet another example of such a procedure is surgical ablation used to treat atrial fibrillation. This condition in the heart causes abnormal electrical signals, known as cardiac arrhythmias, to be generated in the endocardial tissue resulting in irregular beating of the heart.
The most frequent cause of cardiac arrhythmias is an abnormal routing of electricity through the cardiac tissue. In general, most arrhythmias are treated by ablating suspected centers of this electrical misfiring, thereby causing these centers to become inactive. Successful treatment, then, depends on the location of the ablation within the heart as well as the lesion itself. For example, when treating atrial fibrillation, an ablation catheter is maneuvered into the right or left atrium where it is used to create ablation lesions in the heart. These lesions are intended to stop the irregular beating of the heart by creating non-conductive barriers between regions of the atria that halt passage through the heart of the abnormal electrical activity.
The lesion should be created such that electrical conductivity is halted in the localized region (transmurality), but care should be taken to prevent ablating adjacent tissues. Furthermore, the ablation process can also cause undesirable charring of the tissue and localized coagulation, and can evaporate water in the blood and tissue leading to steam pops.
Currently, lesions are evaluated following the ablation procedure, by positioning a mapping catheter in the heart where it is used to measure the electrical activity within the atria. This permits the physician to evaluate the newly formed lesions and determine whether they will function to halt conductivity. It if is determined that the lesions were not adequately formed, then additional lesions can be created to further form a line of block against passage of abnormal currents. Clearly, post ablation evaluation is undesirable since correction requires additional medical procedures. Thus, it would be more desirable to evaluate the lesion as it is being formed in the tissue.
A known method for evaluating lesions as they are formed is to measure electrical impedance. Biochemical differences between ablated and normal tissue can result in changes in electrical impedance between the tissue types. Although impedance is routinely monitored during electrophysiologic therapy, it is not directly related to lesion formation. Measuring impedance merely provides data as to the location of the tissue lesion but does not give qualitative data to evaluate the effectiveness of the lesion.
Another approach is to measure the electrical conductance between two points of tissue. This process, known as lesion pacing, can also determine the effectiveness of lesion therapy. This technique, however, measures the success or lack thereof from each lesion, and yields no real-time information about the lesion formation.
Thus, there is a need for a catheter capable of measuring characteristics of lesion formation in real-time, and doing so with optical imaging, whether the catheter is parallel, perpendicular or at an angle to the tissue. It would be desirable for the catheter to be adapted for ablation as well. To that end, the catheter tip should be transparent yet also electrically conductive so that optical data can be sensed by the catheter tip during, before or after ablation.
There are available many transparent, electrical conductors but each has its limitations. Carbon nanotube film is one such transparent, electrical conductor. Carbon nanotubes were discovered in or about 1991, but their existence had been suspected earlier based on mathematical calculations. Carbon nanotubes have a large length to diameter ratio and thus can be seen as nearly one-dimensional forms of fullerenes. They possess interesting electrical, mechanical and molecular properties. There are single walled nanotubes (SWNT) where the length to diameter ratio is about 1000. There are multi-walled nanotubes (MWNT) with multiple concentric SWNTs with different diameters. MWNTs have different lengths and diameters from SWNTs and they also have different properties.
It is now possible to fabricate ultrathin, transparent, optically homogenous, electrically conducting films of carbon nanotubes and to transfer those films onto various substrates. The challenge had been to deposit nanotubes in a layer thin enough to be optically transparent while maintaining electrical contract through the layer. The films exhibit optical transmittance in the visible spectrum and the infrared. In the near-to-mid infrared, carbon nanotube films have been shown to have good to high transparency for given electrical conductivity of most things currently available. Even in the visible spectrum, the electrical conductivity of nanotube films for given transparency is comparable to commercially available indium tin oxide (ITO) which is another substance that has electrical conductivity and optical transparency.
Accordingly, it would therefore be desirable to provide a catheter that is adapted for optical imaging and electrical conductivity such as for ablation, having a tip that is optically omnidirectional and constructed of carbon nanotube film. Such a catheter may also be adapted for ultrasound imaging concurrently with electrical ablation therapy.